Correct typo 'an historical' → 'a historical'

Author: taboege

doc/Language/glossary.pod6

@@ -148,7 +148,7 @@ X<|Apocalypse>
 
 A document originally written by L<#TimToady>, in which he processed the
 initial barrage of RFCs that came out of the Perl community.  Now only kept
-as an historical document for reference.  See also L<#Exegesis> and
+as a historical document for reference.  See also L<#Exegesis> and
 L<#Synopsis>.
 
 X<|Arity>